The present invention relates to a method of forming an insulating film for use in a semiconductor electronic device, and in particular, to a method of forming an insulating film with few hydrogen atom inclusions for use in the gate of an insulated gate type field-effect transistor (FET).
Developing the fineness of MOS (Metal Oxide Semiconductor) FETs such as VLSI (Very Large Scale Integration)s largely depends on the progress of a technique for forming a gate insulating film that is thin and thermally stable. This is because the reliability and performance of the device largely depends on the characteristics of the gate insulating film. As a gate insulating film, silicon thermal-oxidation film (SiO.sub.2) has been conventionally used. However, a high electric field on the order of MV/cm is normally applied to the gate oxide film, and therefore, a dielectric breakdown due to the high electric field, generation of a surface state due to a hot carrier and a characteristic deterioration due to carrier capture and so forth emerge as problems. In particular, the gate oxide films of the recent MOSFETs are required to have a very thin oxide film having a thickness of not greater than 10 nm, and the aforementioned problems become more serious. In the case of a gate oxide film having a thickness of not greater than 5 nm, not only the film quality is deteriorated by the influence of a natural oxide film but also film thickness control becomes hard to be achieved. As described above, the reduction in thickness of the gate oxide film is coming near the limit.
In an attempt to overcome the limit of the oxide film, a nitriding method with ammonia (NH.sub.3) and an oxy-nitriding method with a compound gas of nitrogen and oxygen, such as nitrogen monoxide (NO), dinitrogen oxide (N.sub.2 O) or nitrogen dioxide (NO.sub.2) have been examined. According to the former method, a nitrided oxide film having a finer structure than the oxide film is formed by heating an oxide film at a high temperature in an ammonia atmosphere, thereby improving a stress resistance. However, according to the method for executing nitriding with ammonia, a great quantity of hydrogen (H) is taken into the oxide film together with nitrogen (N), so that chemical species such as Si--H and Si--N--H are generated. These chemical species operate as an electron trap to vary the threshold voltage of MOSFET. This becomes a problem in terms of a long-term reliability of the device. As a method for suppressing the problem of ammonia with the effect of developing the fineness by nitrogen, a method for forming an oxy-nitrided film having a fine structure by heating an oxide film at a high temperature in an atmosphere of nitrogen monoxide (NO), dinitrogen oxide (N.sub.2 O), nitrogen dioxide (NO.sub.2) or the like has been disclosed by Makoto Yasuda, Nagaru Fukuda et al. in the document of Japanese Patent Laid-Open Publication No. HEI 5-160114. This method can improve the stress resistance without increasing the chemical species such as Si--H and Si--N--H in the oxide film, and therefore, the method is regarded as being promising as a highly-reliable insulating film forming technique as compared with the former method using ammonia.
On the other hand, there has been an attempt to thin the gate oxide film of MOSFET for the purpose of increasing the operating speed and reducing the power consumption of VLSIs, where a gate leak due to direct tunneling becomes significant in the film thickness dimension of not greater than 3 nm. As a method for avoiding this, a method for reducing the gate area by developing the fineness has been reported by H. S. Momose et al. in International Electron Devices Meeting Technical Digest (Pages 593-597, 1994). A method for allowing the gate capacity to be large with a high-dielectric film such as a nitride film even when the insulating film thickness is great has been proposed, for example, by X. J. Wang et al. in VLSI Technology Symposium Digest (Pages 109-110, 1995).
However, in the case where a gate insulating film is oxy-nitrided by a compound gas atmosphere of nitrogen and oxygen, such as nitrogen monoxide (NO), dinitrogen oxide (N.sub.2 O) or nitrogen dioxide (NO.sub.2) for ensuring the reliability of the gate insulating film, the oxidation and nitriding concurrently progress. Accordingly, there is not only the problem that the film thickness and intra-film nitrogen density cannot be independently controlled but also the problem that the film is hard to be nitrided since the intra-film nitrogen density is hard to be increased. When ammonia is used for the nitriding, a great quantity of hydrogen (H) is taken into the oxide film together with nitrogen (N), and consequently chemical species such as --H radical are generated. These chemical species operate as an electron trap to vary the threshold voltage of MOSFET. This has the problem that it degrades the long-term reliability of the device.
When the reduction in thickness of the gate oxide film progresses to a film thickness of not greater than 3 nm, a large gate capacity can be secured by using a nitride film while suppressing the gate leak due to the direct tunneling. However, nitriding with ammonia or CVD with ammonia and silane-based gas is used in this case, and this has led to the problem that a great quantity of hydrogen is taken into the film, forming a great quantity of electron traps and surface states. Particularly when a great quantity of electron traps exist, a current due to Poole-Frenkel conduction flows, and this has caused an increase in gate leak.
Not only the gate insulating film but also insulating films such as a capacitor insulating film have had the problem that the device characteristics are deteriorated by the leak current, electron trap, surface state and so forth.